Add Code Climate badge to README

Hello,

It looks like someone (maybe you) added RABL to Code Climate a little while back, and I thought you might want to know.

I work at Code Climate and we do automated code reviews for Ruby using static analysis. It's free for OSS, and your metrics live here:

https://codeclimate.com/github/nesquena/rabl

In case you want this information to be available to contributors, this PR just adds a dynamic badge to the README (similar to Travis and Gemnasium) that displays the code quality GPA

If you have any questions, let me know.

I hope you find Code Climate useful.

Best -

Noah